1. Field of the Invention
The invention relates to a method and apparatus by means of which a sample can be taken from a process and be analyzed on-line with a capillary electrophoresis apparatus.
2. Description of the Related Art
Electrophoresis is an electrochemical method by which electrically charged particles, and by certain specialized methods also uncharged particles, in an electrolyte solution can be separated; the sizes of the particles may range from the smallest ions and molecules to colloidal particles. Depending on their electric charges and other properties, particles travel at different velocities in an electric field.
In capillary electrophoresis, the background solution travels in a thin tube, capillary, the viscous forces of the liquid preventing convection. The inner diameter of the capillary is usually between 0.02 and 1 mm. Electrophoresis is thus carried out in a free solution, whereby any interferences caused by a carrier are eliminated. It is also easy to remove from the capillary any thermal energy generated by the electric current, and thus a strong electric field can be used, a factor which speeds up separation. Furthermore, capillary electrophoresis is easy to automate.
In capillary electrophoresis, two vessels containing a background electrolyte solution are connected by a capillary tube containing the same solution. Each vessel is equipped with an electrode. The sample to be analyzed is placed as a short zone at the upstream end of the capillary. In general, for the feeding in of the sample, the end of the capillary is transferred from the background solution vessel to the sample vessel and back. This operation causes interferences and distortions in the background solution in the area of the capillary end and in the sample zone and reduces the precision of the method. It is also necessary to switch off the current for the duration of the transfer of the capillary from one vessel to the other, which may cause changes in the conditions of the run. The same disadvantages are caused if the background solution is changed during a run.
The reactions occurring at the electrodes also change the composition of the solution in the background solution vessels, and these changes may pass into the capillary, causing distortions in the parameters of the test series.
Several researchers have presented apparatus options by means of which the disadvantages stated above can in part be eliminated. Virtanen, Acta Polytechnica Scandinavica, Chemistry Including Metallurgy Series, No. 123 (1974), pp. 1-67, as early as the 1960s used an injection technique that allowed the injection of a sample while the electric current was on. Verheggen et al., J. Chromatogr., 452 (1988), pp. 615-622, and Zare et al., U.S. Pat. No. 5,141,621, have also presented a method for the injection of a sample into a capillary electrophoresis apparatus without switching off the electric current. However, these methods and apparatuses do not provide means for exploiting the numerous possibilities provided by the theoretical uniformity of various electrophoresis applications.
Finnish patent FI 103438 discloses a capillary electrophoresis apparatus wherein the carrying out of a certain electrophoresis application requires the selection of certain initial and limit conditions. The control of the limit conditions in a capillary electrophoresis system means that the composition of the background solution in the vicinity of the capillary ends has to be controlled. According to the patent, this is carried out by continuous pumping of fresh solution past the ends of the separation capillary. Thereby the passing of the reaction products, formed in the electrode reactions, into the capillary is also prevented. In order to avoid a high consumption of the background solution, the volume of the solution ducts has to be minimally small. The design of the apparatus according to the present invention is based on this principle. In the apparatus according to the patent, the test conditions can be selected without limitation, and they can be changed freely during a run. However, this arrangement has the disadvantage that it cannot be linked directly to the process but the feeding in of a sample has to be carried out as a separate function.